This invention relates to a contact material for use in a vacuum circuit interrupter, and which is superior in current interrupting performance and breakdown voltage.
Vacuum circuit interrupters have been widely used because they are largely maintenance free, pollution free, provide superior interrupting performance, etc. With such interrupters, a large interrupting capacity and a high breakdown voltage are required. The ability to meet these requirements depends largely on the type of contact material employed.
Desirable properties of the contact material used for vacuum circuit interrupters include a large interrupting capacity, high breakdown voltage, small contact resistance, low melt bonding, small contact erosion, small chopping current, good reproducibility, high mechanical strength, etc.
It is very difficult to provide a material which simultaneously satisfies all of these properties, and practical materials satisfy only specific properties necessary for a specific application, sacrificing other properties to some extent.
U.S. Pat. No. 3,379,846 discloses a contact material which is prepared by melt-diffusing a reactive metal such as Zr or Ti and a high purity metal such as Co, Ag or Au into a sintered refractory metal such as W, Mo, Re, Nb or Ta. U.S. Pat. No. 3,859,089 discloses similar materials.
Coppper-bismuth (Cu-Bi), copper-cobalt (Cu-Co), copper-chromium (Cu-Cr), copper-cobalt-bismuth (Cu-Co-Bi), copper-chromium-bismuth (Cu-Cr-Bi) and copper-beryllium (Co-Be), etc. have been used widely as contact materials in view of total performance. CuBi is a non-solid solution of copper which exhibits a high electric conductivity. The amount of bismuth, which is a low-melting point metal and which forms substantially no solid solution with copper, is equal to or larger than a solid solution limit thereof. Although this combination exhibits a good interrupting performance and an anti-melting adhesion capability, the breakdown voltage thereof is considerably low. Specifically, because copper has a high melting point and bismuth is subjected to evaporation and scattering at times of connecting or interrupting a large current and in high voltage applications when contacts are in the open state, the breakdown voltage is low, leading to a degradation of current interrupting performance over time.
U.S. Pat. No. 4,302,514 discloses a contact material composed of copper in which at least one of Cr, Fe and Co is uniformly dispersed with the particle size of the latter being in a range of 80 to 300 .mu.m or in a range of 30 .mu.m or smaller. However, when this material is used to form contacts, the material tends to evaporate at high temperatures in the vacuum container and hence to be deposited on the walls of metal shields and insulating members, resulting in a reduction of the breakdown voltage of the interrupter. Therefore, materials of this kind make the interrupting current large, and thus such materials are not suitable to form contacts of an interrupter for which a high breakdown voltage performance is required. Further, if the amount of copper thereof is larger than a certain value and if the particle size of the dispersed metal is appropriately selected, the interrupting performance is also superior, and therefore such materials have frequently been used for high-voltage, large-current interrupters. However, the anti-melt bonding performance thereof is relatively poor.
Cu-Co-Bi, Cu-Cr-Bi, etc. have intermediate properties between the above mentioned binary combinations. That is, both of these ternary combinations exhibit relatively superior breakdown performance and interrupting performance and further exhibit superior anti-melt bonding properties due to the presence of Bi. Therefore, such ternary combinations have been used widely. However, since they contain a low melting point metal, the maximum current and voltage which can be applied thereto are necessarily limited.
Therefore, none of the above-discussed materials is fully satisfactory to meet recent demands for higher performance. Because the performance requirements for interrupters could hitherto not be met by selection of contact material, it has been usual to select the configuration of the contact and/or specially design current passages in the contact portion so that a specific magnetic field is generated therearound by which a large current arc is forcibly driven to thus improve the interrupting performance to some extent. The latter approach, however, is still insufficient to meet increasing requirements of higher voltage and large current handling capability, and thus superior contact materials are still a pressing need.